(a) Resonance occurs when one vibrating body sets other body into vibration at its own natural frequency as a result of impulse received from the first body. (ii) Simple experiment to illustrate resonance. The apparatus is arranged as shown. A cylinder is 3/4 filled with water and a tube is dipped into the water so that the length I can be varied. A tuning fork f is set vibrating just above the tube but not touching it. As the fork is set vibrating the length I is gradually increased. A point is reached when a large sound is heard. At this point resonance has occurred because the vibrating tuning fork has set the air in the tube vibrating at its own frequency. (b) Parallel light rays are reflected by a concave mirror on a piece of carbon paper. The converging of the rays on the carbon paper will cause it to ignite. The object O is kept on the centre of curvature of a concave mirror. An exact but inverted image I is formed at the centre of curvature where the screen should be placed.